1. Field of the Invention
The present invention relates to a method for cleaning exhaust gas discharged from internal combustion engines of automobiles or factory boilers, for example, using a catalyst having improved heat resistance.
2. Description of the Background
Various methods have been investigated for eliminating noxious components from exhaust gases discharged from internal combustion engines of automobiles, for example. A conventional method for eliminating noxious components from the exhaust gases is to bring the gases into contact with a catalyst, for example, to effect catalytic reduction. This method requires a reducing agent such as ammonia, hydrogen or carbon monoxide, for example, and additionally requires the use of special equipment for recovering or decomposing the unreacted reducing agent. On the contrary, a catalytic decomposition method allows the elimination of noxious components, especially nitrogen oxides, from the exhaust gases without a special reducing agent and only by passing the exhaust gas through a catalyst layer. This method is most advantageous in view of the simplicity of the process. A catalyst used in this process is a crystalline aluminosilicate catalyst containing copper and having a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio ranging from 20 to 100. See Japanese Laid-Open Patent Application No. Sho 60-125250.
However, the use of the aforementioned exhaust-gas-cleaning catalyst composed of a crystalline aluminosilicate containing copper ion involves the disadvantage of significant deterioration of the activity at a high operating temperature: namely, a remarkable decrease in catalyst activity, after contact with a high-temperature exhaust gas.
Also, there has been an increasing demand for the use of diluted gasolines for combustion in order to lower the fuel cost or reduce the exhausted carbon dioxide gas from internal combustion engines. In this case the exhausted gas contains excessive oxygen for which the above-mentioned conventional "three-way" catalysts cannot be used and, moreover, no practical method has been established for eliminating noxious components.